Apparatus and method of metal extrusion



June 17, 1969 'J. D. MCALLAN 3, 35

APPARATUS AND METHOD OF METAL EXTRUSION Filed Sept. 16. 1966 heet 1 or 4June 17, 1969 v J. D. MCALLAN 3,449,935,

APPARATUS AND METHOD OF METAL EXTRUSION Filed Sept. 16, 1966 Sheet ,2 of4 June 17, 1969 J. D. MOALLAN 3,449,935

APPARATUS AND METHOD OF METAL EX'IRUSION Sept. 16, Sheet 3 of 4 June 17,1969 J, D. MCALLAN 3,449,935

APPARATUS AND METHOD OF METAL EXTRUSION Filed Sept. 16, 1966 Sheet of 4United States Patent 3,449,935 APPARATUS AND METHOD OF METAL EXTRUSIONJohn Dewar McAllan, Glasgow, Scotland, assignor to National ResearchDevelopment Corporation, London, England Filed Sept. 16, 1966, Ser. No.579,901 Claims priority, application Great Britain, Sept. 17, 1965,39,796/65 Int. Cl. B21d 22/10, 25/18; B21c 33/00 US. C]. 7260 18 ClaimsABSTRACT OF THE DISCLOSURE Apparatus and method for extruding a metalbillet of unrestricted length wherein the apparatus includes a tubularram arranged to receive the billet with a small clearance, a diedisposed in an extrusion chamber, means for applying radial pressure tothe ram to cause it to grip the billet, a jack operatively connected tothe ram, means for applying axial pressure to the jack to cause it toadvance the ram and billet to promote extrusion of the billets throughthe die, means for applying fluid pressure to the extrusion chamber of avalue suflicient at least to prevent outward plastic flow of the billetbetween the ram and the entrance to the die under the axial stressproduced by the ram, means for applying axial pressure to the jack inthe reverse direction at a time when radial pressure is no longerapplied to the ram whereby the ram moves backwards over the billet andmeans for holding the billet stationary during the backward movement ofthe ram. A storage chamber may also be provided into which the extrudedmetal issues, and a die may be located at the opposite end of thestorage chamber and means may be provided for applying fluid pressure tothe storage chamber whereby a second stage of extrusion takes placetherefrom which may be intermittent or continuous.

The present invention relates to the extrusion of metals and moreparticularly to apparatus and a method by means of which a metal billetof unrestricted length can be extruded either intermittently orcontinuously.

In the known arrangements by which extrusion is carried out by hydraulicmeans, the billet is surrounded by a high pressure liquid so that itextrudes through a die due to hydrostatic pressure. Each operationnecessitates preparing the billet by cutting it to an appropriatelength, chamfering one end in readiness for the inlet of the die andsealing the billet into a container, all of which, even with a breechmechanism in the latter instance, is timeconsuming, and precludes aproduct of unrestricted length.

The chief object of the invention is to avoid these repeated setting-upsteps, also to achieve higher net extrusion ratios than are presentlypossible hydrostatically and to produce a continuous product such aswire.

According to the invention, apparatus for extruding a metal billet ofunrestricted length comprises a tubular ram arranged to receive thebillet with a small clearance, a die disposed in an extrusion chamber,means for applying radial pressure to the ram to cause it to grip thebillet, a jack operatively connected to the ram, means for applyingaxial pressure to the jack to cause it to advance the ram and billet topromote extrusion of the billet through the die, means for applyingfluid pressure to the extrusion chamber of a value suflicient at leastto prevent outward plastic flow of the billet between the ram and theentrance to the die under the axial stress produced by the ram, meansfor applying axial pressure to the jack in the reverse direction at atime when radial pressure is no longer applied to the ram whereby theram moves backwards over the billet and means for holding the billetstationary during the backward movement of the ram. 1

There may be provided in addition to the above ap paratus a storagechamber into which the extruded metal issue, a die at the opposite endof the storage chamber and means for applying fluid pressure to thestorage chamber whereby a second stage of extrusion takes placetherefrom, which may be intermittent or continuous.

The exit from the die at the entrance to the storage chamber ispreferably provided with a tubular projection of substantially the sameinternal diameter as the ex truded product, this projection contractingon the product and clamping it firmly if the pressure in the storagechamber exceeds that on the other side of the said die, thus isolatingthe storage chamber so that, for example, the second stage of extrusionmay take place whilst the first stage ram is being retracted.

The invention also relates to a method of extruding a metal billet ofunrestricted length comprising the steps of feeding the billet into atubular ram of slightly greater diameter than the diameter of thebillet, applying radial fluid pressure to the tubular ram to cause it tocontract to grip the billet, advancing the ram and hence the billet toproduce extrusion through a die, removing the radial pressure todisengagethe billet from the ram and retracting the ram while holdingthe billet stationary. Preferably the billet between the tubular ram andthe die is subjected to a fluid pressure of a value at least sufiicientto prevent outward plastic flow of the billet under the axial stressproduced by the ram. Before this pressure can be applied the nose of thebillet must be pressed into the entry orifice of the die so as toprevent the escape of the pressurised fluid through the die.

Various embodiments of the invention will now be described in detail, byway of example only, with reference to the accompanying drawingscomprising FIG- URES l6.

FIGURE 1 is a side elevation, partly in section, of metal extrudingapparatus according to the invention;

FIGURE 2 is a perspective view of a core bar with part of a tubular ramand the billet extending therethrough;

FIGURE 3 illustrates diagrammatically the addition of a second stage ofextrusion;

FIGURE 4 is an enlarged view of a die assembly having a tubularprojection assumed to extend into a storage chamber as shown in FIGURE3;

FIGURE 5 is a view similar to that of FIGURE 4 illustrating a coilermechanism applied to the die assembly of FIGURE 4;

FIGURE 6 is a modified arrangement on the lines of FIGURE 1 but with amovable second die whereby the necessary high pressures are generatedwithin the equipment and only a low pressure external source isrequired. Referring to FIGURE 1, a tubular ram 11! 1s fixedly attachedthrough a flange 13 to a jack 11. A circular section billet 12 ofunrestricted length is fed into the tubular part of the ram. Billetsmeasuring in diameter and 12' in length have been used successfullyduring experiments. Lengths may generally be attached to one anothersuccessively if any given length is not suflicrently long for the job inhand, or the extrusiommachme may be fed from a plant continuouslyproducing billet material. Surrounding a portion of the tubular ram is agripper chamber 14 adapted to receive fluid under pressure to providethe required grip by the ram on the billet by contracting the tubularpart thereof on the billet prior to the extrusion of the foremost partthereof through die Disposed within the gripper chamber is a supportingmember 17, referred to as a core bar, and shown in greater detail inFIGURE 2. This member, which is fixed in the assembly, serves to guideand support the ram against any tendency to buckle when driven forwardto extrue the billet. The core bar extends over a substantial part ofthe length of the ram and, because of longitudinal slots 18 disposedtherein and extending formost of the length of the bar, a small quantityof pressurised flurd, conveniently oil, reaches the surface of the ramto cause it to contract and also facilitates its movement through thecore bar. The core bar is in equilibrium, being surrounded by the highpressure fluid and thus has no tendency to contract thereon whenpressure is applied in the gripper chamber. The core bar is slightlyrelieved 1n the region of the slots so that fluid in the gripper chambercan flow around it and thus enter the slots and proceed to the surfaceof the ram. The disposition of the core bar in this chamber reduces thevolume to a few cubic inches, which when occupied by high pressurefluid, is sufl'lcient to maintain the ram in gripping contact with thebillet in spite of the axial force required to promote extrusion throughthe die 15. The shrink fit then occurring between the ram and thebillet, possibly assisted by a seal (such as seal 8 in the embodimentillustrated in FIGURE 3 which is to be later described), prevents fluidfrom escaping from a chamber 16 which is described below.

A pressure chamber 33 defined by the outer casing 41 of the apparatusand the end of the jack 11 has a duct 34 leading thereto through whichfluid under pressure from a fluid pressure supply reservoir (not shown)provides the force required to advance the jack and attached ram axiallytowards the die. Fluid seals 37 are provided to prevent leakage betweenthe jack and the outer casing 41. This outer casing also defines withthe jack a fluid pressure chamber 35 on the opposite side of the jackhead through which pressure fluid is applied by way of a duct 36 towithdraw the ram during the retracting phase of each cycle of operation.A head 42 is attached to the casing 41 by screws one of which is shownat 43. The head 42 is assumed to be secured by the ring 44 to thecontainer 45 embodying both the gripping chamber 14 and the extrusionchamber 16 which is immediately in front of the die 15. Fluid underpressure is introduced into this chamber through a duct 20 which ispreferably connected to a pressure intensifier (not shown) adapted todeliver a constant high pressure during the forward stroke of the ram.

Owing to the small clearance between the ram and the billet, when thepressure on the ram is released and the ram is drawn back to repeat theforward stroke, it tends to drag the billet back with it. To ensure thatthis movement shall not take place, a back stop 38 is provided. Thisstop conveniently consists of a collet chuck with gripping jaws of thetype usually used as a lathe attachment and is operated through a lever39 from cylinder 40 controlled by fluid pressure.

The assembly is held together by two end plates, that 4 at the rearbeing shown at 46. These plates are connected by four parallel boltssuch as 47 extending be tween corresponding corners, the bolts beingprovided with nuts such as 48. The plates are bored centrally, the frontone to admit the billet to be extruded and the back one to permit theproduct of the extrusion to issue forth from the apparatus.

The detailed operation of the apparatus is as follows:

A billet 12 is inserted into the apparatus from left to right and passesthrough the back stop 38, the jack 11 and the tubular ram 10, coming torest against die 15. Fluid under pressure is introduced through duct 19to the gripper chamber 14 causing the ram to contact on the billet.Fluid pressure is applied to the chamber 33 to advance the jack 11 andmove the ram and hence the billet tightly against the die 15 to seal theextrusion chamber 16. High pressure fluid is now introduced into theextrusion chamber 16 through the duct 20, the jack 11 is advanced andextrusion takes place.

The hydrostatic pressure in the extrusion chamber must be at leastsuflicient to prevent outward plastic flow of the billet between the endof ram 10 and the entrance to die 15 but is preferably sufiicient topromote hydrostatic extrusion. As shown in FIGURE 1, the extruded metalemerges from the die at 25.

In order that the ram may make a return stroke after a working stroke,the pressure in the extrusion chamber 16 must be released, this beingfollowed by the release of the pressure in gripper chamber 14 to relaxthe grip of the ram on the billet. To prevent any tendency for thebillet to move backwards, the back stop is now operated to grip thebillet and maintain it steady d ring the back stroke of the ram. The ramis then caused to slide back over the billet by the application of fluidpressure through duct 36 to act on the jack 11. It will be noted thatthe area on which this pressure operates is very much smaller than forthe forward stroke since a much smaller force is needed.

For true hydrostatic extrusion, the high pressure fluid in the extrusionchamber 16 must subject the billet t0 inward stresses exceeding theyield stress of the billet material and the axial stress on the billetshould at least equal the radial stress produced by the high pressurefluid. Nevertheless if a greater extrusion ratio is desired,satisfactory extrusion of the billet through the die can take place withthe axial stress greater than the radial stress so long as thedifference does not exceed the yield stres of the material.

The gripping pressure should not exceed the elastic limit of thematerial so that the surface of the portion of the billet not extrudedremains unmarked. Holding the material without spoiling its surface isconsidered desirable by many manufacturers. This etfect is obtained byusing gripping pressures such as to not permit the gripping force toexceed the elastic limit of the billet material.

Referring now to FIGURE 3 of the drawings, when it is desirable toemploy a second stage of extrusion to provide further reduction in thecross-sectional area of the product, there is provided, subsequent tothe die 15, additional apparatus for this purpose. This apparatuscomprises a generally cylindrical storage chamber 21 in which theproduct 25 is collected and a projection member 22 for directing theproduct into the chamber. This member, shown in detail in FIGURE 4,comprises a die 23 having a tubular projection 24 extending into anante-chamber 26. This tubular projection may be integral with the die orattached thereto. The storage chamber 21 may be at atmospheric pressureor at some back pressure during extrusion through the die 15. On raisingthe pressure in storage chamber 21 the tubular projection 24 will gripthe product 25 from the die 23 thus avoiding any tendency for backwardmovement and extrusion will take place through die 31.

If back pressure in storage chamber 21 is suificient to causehydrostatic extrusion through die 31 and if the pressure in chamber 16exceeds the pressure in storage chamber 21 by the extent necessary toextrude material through die 15, the first product from die 31 may becaused to emerge continuously notwithstanding the fact that extrusionthrough die is intermittent.

When the product 25 issues into the chamber 21, it tends to form a coiltherein as shown in somewhat idealised form in FIGURE 3, so that anappreciable amount of the first extrusion product may be stored. Topromote this operation in an orderly manner, the arrangement of FIGURE 5may be employed. This incorporates in the ante-chamber 26 a coilermechanism comprising a reel member 27 rotatable on needle roller bearing28 and co-operating with an anti-friction thrust ring 29. The member 27has an extension 30 provided with a bore extending from the centre ofthe member 29 of the same diameter as the tubular projection 24 anddisposed at a slight angle to the axis of the chamber 21, Thisarrangement provides that the product 25, on leaving the tubularprojection 24, is directed by the reel 27 into the chamber 21 at anangle to the axis of the chamber and additionally with a component ofdirection tangential to chamber 21 and will therefore coil therein morereadily. As the product coil in the chamber 21, it rotates the reel 27.

A second die 31 positioned at the exit of the chamber 21, FIGURE 3,co-operates in the further and continued extrusion of the metal coiledwithin the chamber. When fluid pressure is applied in the chamber by wayof the duct 32, the coiled metal extrudes through this die. Beingusually much reduced in gauge at this stage, the second extrusionproduct may be wound on to a spool, or if a finer gauge product isrequired, the wire is directed through a further storage chamber and dieor series of chambers and dies.

When a plurality of storage chambers are employed, the pressures in thechambers may be adjusted in such a manner that simultaneous extrusioncan be obtained from alternate chambers, all the chambers being providedwith tubular projection members and preferably also withcoiler-mechanism. Difficulty will arise if conditions are such as toresult in underor over-filling of a cylindrical chamber. In the firstinstance the product will become straight within the chamber so thatnecking occurs and probably resultant breakage. This can be avoided bysuitably limiting the pressure in that chamber. Over-filling is lesslikely to occur since when the receiving cylindrical chamber approachesthe full condition, the hydrostatic pressure attains such a value inrelation to that in the previous extrusion chamber that extrusionceases. It is desirable, however, to time the application of pressure tothe extrusion chambers to avoid billet breakage or overfilling of thechamber into which extrusion is taking place.

The arrangement described in FIGURE 1 depends for its operation on thesupply of high pressure fluid from an external source, and this is fedinto a central bore at different places, particularly to the extrusionchamber and the gripping chamber. The ducts whereby this is doneconstitute a source of weakness in the main frame and it is thereforepreferable for this to be eliminated and for the whole device to beoperated from a comparatively low external pressure, the required highpressure being generated therefrom in the device itself.

A modified construction which enables this to be done is shown in FIGURE6 which indicates in line diagram form the relationship of the variousparts.

A before, the main body 51 within which extrusion takes place is veryheavily constructed to resist the high pressures concerned and it isprovided at one end with a cylinder 52 of much lighter construction anda similar cylinder 53 at the other end. Working within the cylinder 52are two pistons 54 and 55, 54 carrying the tubular ram 56, while 55 hasan extension 57 closely fitting the bore in the main body. The cylinder55 is provided with the piston 58 mounted on a hollow rod 59 carrying adie 60 at its left-hand end. The space between this die and the main die61 forms a storage chamber in which the first extrusion product throughthe main die tends to coil. The die 61 is arranged to act as a valve inthat it is movable on its seat to permit fluid flow past it when thepressure on the right-hand side exceeds that on the left-hand side. Asin the previous arrangement, a backstop 62 is provided, the function ofwhich is to hold the billet 63 during the return stroke of the ram.

The interaction of the various parts will be followed from thedescription of a typical cycle. At this point in the cycle the backstop62 is disengaged by control arrangements not shown to enable the billetto pass freely therethrough. This piston 55 is now moved to the right bypressure applied to the space 64. In view of the comparatively largediameter of the piston 55, this pressure can be comparatively low butwill serve to produce in due course the necessary high pressure in thegripping chamber 65. The movement of the piston 55 will cease when thispressure has been attained and as a result the ram 56 is caused tocontract so as to grip the billet 63. The pressure in space 64 ispreferably maintained automatically at a predetermined maximum value bythe provision of a suitable relief valve, not shown. This is desirablein view of subsequent movements of the pistons 54 and 55. Piston 58 atthe other end of the main block is now moved to the left by theapplication of low pressure to space 66 and this causes correspondingmovement of the die 60 and hence extrusion therethrough of the productremaining in the storage chamber from earlier extrusion cycles throughthe die 61.

The increase in pressure in the storage chamber causes the die 61 toshift slightly off it seating and as a result pressure rises in thespace 67 forming the first stage extrusion chamber. Piston 58 is nowretracted by applying low pressure to space 68 and exhausting space 66,and as this movement takes place, the pressure in the storage chamberfalls oif and the die 61 re-seats to maintain the high pressure in thespace 67. The pressure in the storage chamber may be allowed to drop toatmospheric pressure on a suitable value of back pressure.

Piston 54 is now moved to the right by applying pressure to space 69 andas a result the ram 56 and the billet 63 are moved forward and extrusiontakes place through die 61. The magnitude of the pressure applied to thespace 69 determines the axial stress in the billet. For extrusion underhydrostatic conditions this should at least equal the radial stressesdue to the pressure in space 67, but if a higher extrusion ratio isdesired, the axial stress may be arranged to exceed the radial stress solong as it does not do so by more than an amount equal to the flowstress of the billet material. As a result of the forward movement ofthe ram 56, the pressure in space 67 tends to rise but any increase isprevented by the valving action of the seal 70 at the die end of thechamber 65. This has the effect of increasing the pressure in the space65 which causes the piston 55 to move to the left to maintain thegripping pressure at the predetermined level dependent on the constantpressure in the space 64.

At the end of the extrusion stroke, the backstop 62 is engaged to holdthe billet and prevent it backward movement. Piston 55 is now movedbackwards, i.e. to the left in FIGURE 6, by applying pressure to thespace 71 and exhausting space 64. Consequently pressure falls off inspace 65 and also in space 67 so that the gripping pressure is releasedand the pressure in space 64 permits the piston 54 to move to the leftand thus retract the ram to the starting position.

During the course of the cycle, there is a progressive transfer of fluidfrom right to left by way of die 61 and seal 70 which would produce aprogressive movement of piston 55 to the left and interfere with properoperation. This is taken care of, however, by the leak provided by thechannel 72 which is uncovered if the piston 55 is moved sufliciently farto the left and then permits a momentary escape of fluid.

It is important that the piston 54 shall be capable of slight movementto the left from its normal fully-retracted position during the pressurebuild-up at the beginning of the cycle, so that the movement of die 61from its seating is not prevented by the billet acting as a strutbetween die 61 and ram 56, and to ensure this a pair of Bellevillespring washers 73 is provided to prevent the shoulder of piston 54engaging with the end of the cylinder 52 at the end of the retractingmovement.

The arrangement just described for pressurising the extrusion chamberdepends on the movement of the piston 58 generating pressure in thestorage chamber and this cannot take place unless the hole in the die 60is sealed. This will not be the case when a new billet is inserted intothe machine and a special starting operation is therefore necessary.This may consist in the removal of the cylinder 53 together with the die60 to permit the insertion of a plug in the die. The cylinder is thenreplaced and several normal cycles are performed which will producesuflicient extrusion product in the storage chamber. Thereupon thecylinder 53 is again detached and the plug removed from the die 60 andreplaced by the end of the extrusion product. The cylinder 53 is thenreplaced and thereafter normal operation is possible.

As an alternative to detaching the cylinder 53, a sealing cap may beplaced over the end of the rod 59 which will enable suflicientpressurisation to result for extrusion to take place through the die 61and the end of the extrusion product will eventually find its way intothe die 60, possibly with the assistance of a guide associated therewithin the storage chamber.

The present invention may be combined with a continuous casting plantfrom which stock is fed direct. This presents the advantage that thestock can enter the exrustion plant while still hot, so that with manysubstances extrusion is much facilitated and higher extrusion ratios arepossible with a consequent reduced number of extrusion stages.

The sequence of operation in each cycle may be automatically controlledpreferably by a suitable tirner which energises electrically operatedvalves to control the working fluid. After a few trials to determine theoptimum periods, this will enable the process to be automatic,particularly if the billet material is being continuously produced.

I claim:

1. Apparatus for extruding a metal billet of unrestricted lengthcomprising a tubular ram arranged to receive the billet with a smallclearance, a die disposed in an extrusion chamber, means for applyingradial pressure to the ram to cause it to grip the billet, a jackoperatively connected to the ram, means for applying axial pressure tothe jack to cause it to advance the ram and billet to promote extrusionof the billet through the die, means for applying fluid pressure to theextrusion chamber of a value sufiicient at least to prevent outwardplastic flow of the billet between the ram and the entrance to the dieunder the axial stress produced by the ram means for applying axialpressure to the jack in the reverse direction at a time when radialpressure is no longer applied to the ram whereby the ram moves backwardsover the billet and means for holding the billet stationary during thebackward movement of the ram.

2. Apparatus as claimed in claim 1 in which the means for applyingradial pressure to the ram comprises a fluid pressure chamber disposedaround at least part of the ram.

3. Apparatus as claimed in claim 2 in which the means for applyingradial pressure to the ram comprises an annular piston surrounding thetubular ram and movable axially of the ram within the pressure chamber,means for advancing the piston in the direction of the die to pressurisethe pressure chamber, unidirectional sealing means preventing flow offluid from. the pressure chamber to the extrusion chamber but enablingfluid to (flow in the opposite direction past the sealing means torelieve any excess pressure in the extrusion chamber arising from theadvance of the tubular ram into the extrusion chamber in the course ofextrusion, and means for limiting the force applied to the piston toadvance it as aforesaid, whereby it is enabled to retreat when fluidflows from the extrusion chamber past the sealing means into thepressure chamber to relieve excess pressure in the extrusion chamber.

4. Apparatus as claimed in claim 2 in which the pressure chamberincludes a fixed tubular supporting member disposed around the ram witha small clearance therefrom and provided with radial slots to permitfluid in the pressure chamber to contact the surface of the ram.

5. Apparatus as claimed in claim 2 in which the necessary fluidpressures are applied to the different chambers from an external source.

6. Apparatus as claimed in claim 2 in which the necessary fluidpressures in the different chambers are pro duced as a result of theapplication of low pressure to produce movement of a jack ofconsiderably greater diameter than the diameter of the chambers.

7. Apparatus as claimed in claim 1 including a storage chamber intowhich the extruded metal issues, a die at the opposite end of thestorage chamber and means for applying fluid pressure to the storagechamber whereby continuous extrusion takes place therefrom.

8. Apparatus as claimed in claim 7 in which the fluid pressure in thestorage chamber for producing extrusion is less than the pressure in theextrusion chamber during the extrusion operation therefrom.

9. Apparatus as claimed in claim '8 in which valve means are providedbetween the storage chamber and extrusion chamber operable to permit theflow of fluid from the former to the latter when the pressure in theformer exceeds the pressure in the latter but to impede the flow offluid from the latter to the former.

10. Apparatus as claimed in claim 9 in which the die at the entrance tothe storage chamber rests in a seating from which it is movable, wherebyit functions as the said valve means and whereby the extrusion chamberis pressurised when the storage chamber is pressurised.

11. Apparatus as claimed in claim 7 in which the die at the opposite endof the storage chamber is movable axially to raise the pressure in thatchamber to a value suflicient to produce hydrostatic extrusion.

12. Apparatus as claimed in claim 7 in which a tubular projectionattached to the die at the entrance to the storage chamber and of thesame diameter as the neck of the die extends a short distance into thestorage chamber and is arranged to contract under pressure in thechamber on to the extrusion product thus preventing backward movement ofthe extrusion product when pressure is reduced in the extrusion chamberto a level below that of the pressure in the storage chamber.

13. Apparatus as claimed in claim 12 in which a rotatable coiler memberis disposed adjacent the tubular projection, said member having anobliquely disposed bore of a diameter similar to that of the tubularprojection and arranged to receive the product issuing therefrom andassist it in coiling in the storage chamber.

14. A method of extruding a metal billet of unrestricted lengthcomprising the steps of feeding the billet into a tubular ram ofslightly greater diameter than the diameter of the billet, applyingradial fluid pressure to the ram to cause it to contract to grip thebillet, advancing the ram and hence the billet to produce extrusionthrough a die, removing the radial pressure to disengage the billet fromthe ram and retracting the ram while holding the billet stationary.

15. A method as claimed in claim 14 including applying to the billetbetween the ram and the entrance to the die, a surrounding hydrostaticpressure sufiicient at least to prevent outward plastic flow of thebillet under the axial stress applied to it by the ram.

16. A method of extruding a metal as claimed in claim 14 includingdirecting the extruded metal to a storage chamber where it is collected,and applying fluid pressure to the chamber to cause continuous extrusionof the metal through a die at the opposite end of the storage chamber.

17. A method as claimed in claim 16 including gripping the extrudedmetal issuing from the die at the entrance to the storage chamber whenthe pressure in the storage chamber exceeds the pressure on the otherside of the said die.

18. A method as claimed in claim 16 in which the extruded metal isdirected through a series of storage chambers in which the crosssectional area of the billet is reduced successively.

References Cited UNITED STATES PATENTS 524,504 8/1894 Robertson 7254524,506 '8/ 1894 Robertson 7254 1 524,509 8/1894 Robertson 72-541,575,768 3/1926 Judge 72268 2,866,545 12/1958 Honkala 72-264 3,201,9668/1965 Benteler 72264 3,243,985 4/1966 Green 72260 RICHARD J. HERBST,Primary Examiner.

US. Cl. X.R. 72270, 271

